During sports and other physical activity, individuals are often exposed to impact forces that, if not at least partially attenuated, can cause severe injury. Therefore, they usually wear protective sporting gear, such as helmets, shields, elbow and knee pads, etc. Such protective gear typically includes impact-attenuating structures that deform elastically and/or plastically in response to an impact force, thereby mechanically attenuating the impact. For example, many helmets have a crushable foam layer disposed between a rigid or semi-rigid outer shell and an inner liner that conforms the helmet to the wearer's head.
Foams are generally customized to respond optimally to a specific range of impact energies, but outside this range, their effectiveness is significantly reduced. For impact energies exceeding the high end of the range, the foam is too soft and “bottoms out”—i.e., reaches maximum compression—before the impact is fully attenuated, resulting in the transfer of high impact forces to the body. For impact energies below the optimal range, on the other hand, the foam is too hard to compress, or “ride down,” sufficiently to adequately prolong the distance and time over which deceleration occurs following impact, resulting in sudden, high peak forces. The only way to improve the impact-attenuating capability of a foam layer is, typically, to decrease the density of the foam (i.e., make it softer) and increase the thickness of the layer, which results in an undesirable increase in the amount of material used. Exacerbating this trade-off, the maximum ride-down distance for most foams is only about 30-40% of the original height. Thus, about 60-70% of the foam layer add to the bulk and weight, but not the impact-absorption capacity, of the protective structure. In addition, the performance of many foams degrades rapidly with repeated impacts. Other conventional impact-absorbing layers exhibit similar problems and limitations.
More recent helmet designs feature, in place of a continuous layer, discrete compression cells, which attenuate the impact with their side walls and/or by resistively venting a fluid through an orifice of the cell enclosure. These cells generally have ride-down distances close to their height, exhibit superior durability, and adapt to a wide range of impact energies. Furthermore, they provide opportunities for tailoring the impact-absorption characteristics of the helmet (or other protective structure) via the cell design. Such customization opportunities, however, have rarely been exploited.